Mount Prospect, Illinois 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

881
FXUS63 KLOT 211752
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1252 PM CDT Sun Jun 21 2026

.KEY MESSAGES...

- Widespread soaking rainfall is expected across the area
  through tonight. A corridor of locally higher rainfall rates
  may occur near/south of I-80 leading to localized flooding.

- There is a low chance (20-30%) for isolated non-severe
  thunderstorms south of a Pontiac to Rensselaer line this late
  afternoon through early evening.

- A heightened risk for rip currents will exist at southern Lake
  Michigan beaches from this evening through Monday night.

&&

.UPDATE...
Issued at 1053 AM CDT Sun Jun 21 2026

Rain showers are spreading across northwest IL in association
with a pair of shortwave troughs that are moving across IA and
the Dakotas. At the same time, a warm front is being analyzed
near the IA-MO border and extends east-southeast across central
IL into south-central IN. Near the warm front is a line of
showers and thunderstorms with an embedded MCV that is
progressing east-northeast out of northern MO. While recent
radar trends indicate that the MCV should pass just south of our
area through early afternoon, there is a 20-30% chance for some
isolated thunderstorms to develop along and south of a Pontiac
to Kankakee to Rensselaer line between 11 AM and 2 PM.
Otherwise, expect mainly light to moderate rain showers to fill
in across northeast IL and northwest IN through early afternoon.

Heading into this afternoon (after 2 PM), the aforementioned
shortwaves are expected to phase and deepen as they move into IL
which should allow mid-level lapse rates to steepen slightly
(to 6-7 C/km). As this occurs, the warm front will also be
lifting across central IL. The expectation is for the ongoing
showers (and any isolated storms) this morning to keep the warm
front south of a Bloomington to Lafayette line which should
limit the better destabilization and keep the thunderstorm
threat south of a Pontiac to Rensselaer line this afternoon.
However, the broad forcing and modest lapse rates may allow an
isolated lightning strike to occur as far north as the I-88 to
I-80 corridors. There also remains a non-zero chance for a
couple of stronger storms in our southern CWA this afternoon.
Though, with the warm front expected to stay to our south the
main threat with any stronger storms looks to be gusty winds up
to 40 mph.

Regardless, the main concern for this afternoon will be
the increasing rain rates as the mid-level baroclinic zone
establishes overhead and interacts with the modest instability
and forcing from the shortwaves. While the threat for the
heaviest rainfall (totals upwards of 1-2+ inches) remains south
of I-80, the concern remains for a band of heavier showers to
develop between I-88 and I-80 later this afternoon and evening.
If this band of heavier rain does materialize as some guidance
suggests, then a localized flash flooding threat could develop
in the Chicago metro. For this reason most of northeast IL and
northwest IN (namely areas south of I-88) remain under a level
2 of 4 risk for flash flooding. So if you are out and about
this afternoon and evening be on the lookout for ponding and/or
flooding in lower-lying areas and remember to never drive
through flooded roads.

Rain showers are expected to persist through most of the night
before rain gradually tapers from west to east late tonight into
Monday morning. Though, as rain tapers winds will be increasing
out of the east-northeast which will result in building waves
and dangerous rip currents on Lake Michigan through Monday.

Overall the going forecast captures these expectations well so
only minor adjustments where made with the morning updates. All
updated products have been sent.

Yack

&&

.DISCUSSION...
Issued at 342 AM CDT Sun Jun 21 2026

Through Tonight:

GOES satellite and WSR-88D radar mosaic imagery this morning
depict an expansive convective complex across the central Plains
working its way towards the east and southeast. An MCV is
evident in central Kansas at the time of this writing, displaced
to the south of the parent shortwave that helped kickstart this
convection yesterday afternoon as it ejected eastward out of
the central Rockies. As this MCV, shortwave, and an associated
surface low continue to progress eastward today, an expansive
precipitation shield will spread over our forecast area from
west to east from the mid-late morning through this afternoon,
bringing a widespread soaking rainfall to the area that should
last for several hours. This is an aspect of high confidence
with our forecast. Two forecast aspects that remain of lower
confidence, however, are the potential for flooding and
thunderstorms in our area today as this system moves through the
region.

One feature that will play a key role in whether either of
these threats are realized in our area today is a warm front
that was analyzed to be present from near Kansas City to just
north of Jackson, TN at 0800Z. This warm front will effectively
serve as the northward terminus of where a threat for severe
weather will be present today and will also help dictate how far
north the threat for lightning and convectively-enhanced
rainfall rates will trek. The latest CAM guidance, for what it`s
worth (which probably isn`t a whole lot as it, unsurprisingly,
does not have a good handle on the ongoing convection),
generally favors the warm front lifting northward into central
Illinois and Indiana this afternoon and possibly as far north as
our southernmost tier of counties. These CAMs are modeling this
outcome under the assumption that the ongoing central Plains
MCS will slow down and/or decay through the remainder of the
night and into the morning, affording the warm front enough time
to lift this far north before having its progress stalled by
the incoming convection and rain.

While the MCS has notably weakened over the past few hours, the
embedded MCV still appears to be well-defined and is generating
new convective cells out ahead of it, and there is a reservoir
of untapped 1000-2000 J/kg of MUCAPE out ahead of the complex in
southern Missouri, so still have some doubts over whether this
MCS will lose enough steam for the warm front to be able to make
it as far north as the aforementioned CAM solutions are
suggesting. Nevertheless, if the warm front were indeed to make
it as far north as our southern CWA, then rich low-level
moisture paired with strong low-level and effective shear values
resulting from 50+ kt mid-level flow and backed surface winds
near the warm front would yield a threat for low centroid
supercell structures or bowing segments capable of producing
tornadoes and damaging winds in our far southern counties today
(in the area more or less delineated in SPC`s Day 1 Convective
Outlook). That said, the more likely scenario at this time is
that the warm front and the attendant severe weather threat will
remain south to potentially well south of our forecast area as
a result of the incoming central Plains convection arriving at
our longitude faster than indicated in CAM guidance and
accordingly forcing the warm front to remain farther south than
indicated in these models.

Even if the warm front does not quite reach our area, there are
still indications that a layer of meager elevated instability
could claw as far north as the I-80 or I-88 corridor late this
afternoon and evening and pose some lower-end threat for
isolated lightning strikes in our area during this late day time
frame while the main show remains to our south. Additionally, it
still appears that the main synoptic surface low will track
across our southern counties or just to our south across central
Illinois and Indiana. Several models have been depicting
notable frontogenetical forcing along the northern flank of the
surface low inducing a band of precipitation featuring enhanced
rainfall rates. Where this band of precipitation sets up and
just how torrential the rainfall ends up being within it will
depend on the exact strength and track of the surface low,
which, due to the extensiveness of the upstream convection,
remains extremely difficult to pin down at this time.

In a scenario with a stronger surface low and associated
deformation/f-gen band, rainfall rates could be efficient enough
amidst 1.5-2" precipitable water values to cause flooding
somewhere in or very near our forecast area. The latest flash
flood guidance (FFG) values from the North Central River
Forecast Center are generally around 1.5-2" for 3 hours and
2-2.5" inches for 6 hours, and while there are a few exceptions
(e.g. lower values for our more heavily urbanized and flood-
prone areas), those are generally the rainfall thresholds that
we believe could start to cause some more notable hydrologic
concerns to arise if they are exceeded with the specified time
interval(s). Confidence in whether these thresholds will be
exceeded in our forecast area today and where was too low to
warrant the issuance of a Flood Watch with the overnight
forecast package, but couldn`t rule out the need for a short-
fused Flood Watch issuance by the day shift if the forecast
picture clears itself up more later this morning. Either way,
area creeks, streams, and rivers will rise as a result of this
rainfall, and it`s not out of the question that some additional
river flooding could occur somewhere in the near future as a
result of today`s rainfall.


Monday through Saturday:

Some shower activity from Sunday`s storm system may linger
around through Monday morning, but should clear our area by the
early afternoon. During the afternoon, couldn`t rule out
diurnal cumulus growing deep enough to support a few spits of
rain wherever surface convergence/confluence is maximized, but
otherwise, expect a dry day on Monday with highs in the mid 60s
(near Lake Michigan) to the mid 70s (well inland) as surface
high pressure begins to build into the region. Buoyed by breezy
onshore flow, high waves that will develop across southern Lake
Michigan today will persist into Monday and continue to create a
heightened threat of rip currents at area beaches through
Monday night. Winds and waves will subside on Tuesday as the
high pressure center passes close by, while daytime temperature
readings will likely only be a touch milder compared to Monday.

Late Tuesday night into Wednesday, global ensemble and
deterministic guidance remains in good agreement that an upper-
level trough will slide into the Upper Midwest, likely
accompanied by a low pressure system at the surface. Moisture
profiles should be plenty adequate to support rain showers
across the region as this system passes by, and while lapse
rates look to be lackluster on the whole, deterministic GFS and
ECMWF soundings both depict CAPE profiles with equilibrium
levels solidly above -20C that should be favorable for
thunderstorms to occur in the area as well.

Ensemble consistency breaks down over the latter half of the
week into the weekend, but still broadly suggests that a few
follow-up shortwave troughs could traverse the region in the
days following Wednesday. Thus, additional showery and perhaps
stormy periods could be seen going into the weekend, but
confidence in the specific details of that potential remains
low at this time. Confidence is higher in near to below normal
temperatures persisting through the entirety of this upcoming
week, though ensemble guidance does suggest that some warmer
temperatures could be on the horizon just beyond the end time
of our current 7-day forecast period.

Ogorek

&&

.AVIATION /18Z TAFS THROUGH 00Z TUESDAY/...
Issued at 1252 PM CDT Sun Jun 21 2026

Key aviation forecast messages:

- Widespread rain to continue into tonight with periods of MVFR
  to IFR CIGs and VSBYs expected.

- Gusty east to northeast winds may develop late afternoon
  through the evening, with gusts to 25-30 kt near and south of
  I-80.

Steady rain will continue through the rest of the day and into
the early overnight hours across the area. Expect a gradual
lowering of CIGs and VSBYs through mid evening to MVFR and
potentially IFR. An axis of higher rain rates and associated IFR
VSBYs may set up somewhere within the vicinity of the I-80
corridor by early evening. The favored timing of this feature
has shifted from late afternoon to early evening and have
adjusted the inherited TEMPO groups accordingly, with lower
VSBYs (2SM) at MDW and GYY.

Winds may also become increasingly gusty late this afternoon
through the evening out of the east and northeast. Depending on
the strength of the surface low, a narrow corridor of gusts up
to 25-30 kt may materialize, currently favoring areas south of
I-80. Have nudged up gusts slightly for the Chicago area
terminals and included a TEMPO for higher gusts at GYY.

A rogue lightning strike can`t be ruled out within the broader
stratiform rain area but confidence and coverage continues to
be too low for a formal TAF mention (<15% chance), with the
better potential remaining well south of I-80.

Expect the rain to gradually come to an end late tonight
between roughly 7-9Z with lingering IFR to MVFR CIGs through
daybreak that gradually scatter out by mid Monday morning.

Petr

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Beach Hazards Statement from 10 PM CDT this evening through
     late Monday night for ILZ006-ILZ103.

     Beach Hazards Statement from 7 PM CDT this evening through
     late Monday night for ILZ104.

IN...Beach Hazards Statement from 7 PM CDT this evening through
     late Monday night for INZ001-INZ002.

LM...Small Craft Advisory from 4 PM this afternoon to 1 AM CDT
     Tuesday for the IL nearshore waters.

     Small Craft Advisory from 4 PM this afternoon to 4 AM CDT
     Tuesday for the IN nearshore waters.

&&

$$

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